Process and aqueous solution for phosphatizing metallic surfaces

ABSTRACT

A process is disclosed for preparing an aqueous zinc phosphatizing solution for producing phosphate coatings on metallic surfaces of iron, steel, zinc, zinc alloys, aluminum or aluminum alloys, which comprises: 
     (1) 0.3 to 5 g Zn 2+ /1; 
     (2) 0.1 to 3 g nitroguanidine/1 as an accelerator; 
     (3) phosphate where the acid value is 0.03 to 0.3 indicating the ratio of free acid, calculated as free P 2 O 5  to the total P 2 O 5 , and the weight ratio of Zn to P 2 O 5  is 1:5 to 1:30; and 
     (4) balance water, where the solution produces finely crystallite phosphate coatings in which the crystallites have a maximum edge length &lt;15 μm, which comprises the steps of: 
     (a) preparing a concentrate comprising the Zn 2+  and the phosphate in water; 
     (b) supplying additional water to the concentrate so that the Zn 2+  concentration in the phosphatizing solution is 0.3 to 5 g/1; 
     (c) preparing a stabilized suspension consisting essentially of 100 to 300 g of nitroguanidine/1, 10 to 30 g of sheet silicate/1 as stabilizer and the balance water by suspending the sheet silicate in deionized water and stirring the nitroguanidine into the suspension; and 
     (d) introducing the stabilized suspension into the phosphatizing solution so that the nitroguanidine concentration in the phosphatizing solution is 0.1 to 3 g nitroguanidine/liter.

FIELD OF THE INVENTION

This invention relates to an aqueous, phosphate-containing solution forproducing phosphate coatings on metallic surfaces of iron, steel, zinc,zinc alloys, aluminum or aluminum alloys. This invention furthermorerelates to a process for phosphatizing by using an aqueous phosphatizingsolution.

BACKGROUND OF THE INVENTION

From the DE-PS 750 957 there is known a process for improving thecorrosion resistance of metals, in particular of iron and steel, bytreating them in a solution forming phosphate coatings, where thesolution contains an accelerator, and where nitromethane, nitrobenzene,picric acid, a nitroaniline, a nitrophenol, a nitrobenzoic acid, anitroresorcinol, nitrourea, a nitrourethane or nitroguanidine is used asaccelerator. The optimum concentration for the individual acceleratorsis different, but in the phosphatizing solutions it is generally in therange between 0.01 and 0.4 wt-%. For the accelerator nitroguanidine theoptimum concentration is said to be 0.2 wt-%. From the DE-OS 38 00 835there is known a process of phosphatizing metal surfaces, in particularsurfaces of iron, steel, zinc and the alloys thereof as well asaluminum, as a pretreatment for the cold working, where withoutactivation at a temperature in the range from 30 to 70° C. the surfaceis brought in contact with an aqueous solution containing 10 to 40 gCa²⁺/l, 20 to 40 g Zn²⁺/l, 10 to 100 g PO₄ ³⁻/l and as accelerator 10 to100 g NO₃ ⁻/l and/or 0.1 to 2.0 g organic nitro compounds per liter,where the solution has a pH value in the range from 2.0 to 3.8 and aratio of free acid to total acid of 1:4 to 1:100. As accelerator, anm-nitrobenzene-sulfonate and/or nitroguanidine may be used. Thephosphate coatings produced in accordance with the known process havecoating weights of 3 to 9 g/m².

Although it is known per se that nitroguanidine can be used asaccelerator when phosphatizing metallic surfaces, the practical use ofthis accelerator meets with some difficulties, as the phosphatizingresults achieved are very frequently unsatisfactory. This is quiteobviously due to the fact that the effect of the acceleratornitroguanidine very much depends on the inorganic components of thephosphatizing solution and the concentration of the inorganic componentsof the phosphatizing solution, so that the phosphate coatings producedby using nitroguanidine only have good functional properties when onesucceeds in providing a phosphatizing solution in which the individualcomponents are adjusted to each other such that when usingnitroguanidine as accelerator, phosphate coatings of good, constantquality can be produced also in a continuous operation. Moreover, theinteraction between the nitroguanidine and the remaining components ofthe phosphatizing solution cannot be predicted or determined bytheoretical considerations or simple experiments, but must be determinedby extensive experiments on different phosphatizing systems. Thefrequently unsatisfactory results are also due to the poor watersolubility and the uneven distribution of the nitroguanidine.

OBJECT OF THE INVENTION

It is therefore the object underlying the invention to create an aqueoussolution for phosphatizing metallic surfaces, which containsnitroguanidine as accelerator, and whose remaining components areadjusted to each other such that the phosphate coatings formed duringphosphatizing are finely crystalline, have a low coating weight, providefor a good lacquer adhesion and ensure a good protection againstcorrosion. It is furthermore the object underlying the invention tocreate a process of phosphatizing which uses the phosphatizing solutionin accordance with the invention, where the process should operate attemperatures as low as possible, may be used for phosphatizing differentmetallic surfaces, and should operate by using simple technical means aswell as to be safe in operation.

SUMMARY OF THE INVENTION

The object underlying the invention is obtained by preparing an aqueous,phosphate-containing solution for producing phosphate coatings onmetallic surfaces of iron, steel, zinc, zinc alloys, aluminum oraluminum alloys, which contains 0.3 to 5 g Zn²⁺/l, and 0.1 to 3 gnitroguanidine/l, where the acid value is 0.03 to 0.3 and the weightratio of Zn:P₂O₅=1:5 to 1:30, and which produces finely crystallinephosphate coatings, in which the crystallites have a maximum edge length<15 μm. It has surprisingly turned out that by means of thephosphatizing solution in accordance with the invention very finelycrystalline phosphate coatings can be produced, which effect a goodlacquer adhesion and a good protection against corrosion. Thecrystallites have a laminated, cuboid or cubic shape and always have amaximum-edge length <15 μm, which in general is even <10 μm.Furthermore, the phosphatizing solution in accordance with the inventionis very well suited for phosphatizing cavities. The phosphate coatingsdeposited on the metallic articles from the inventive phosphatizingsolution have a coating weight of 1.5 to 4.5 g/m², preferably of 1.5 to3 g/m², so that the lacquer adhesion is favorably influenced. With azinc content >5 g/l the anticorrosive properties and the lacqueradhesion deteriorate significantly.

The ratio of Zn:P₂O₅ is based on the total P₂O₅. The determination ofthe total P₂O₅ is based on the titration of the phosphoric acid and/orthe primary phosphates from the equivalence point of the primaryphosphate to the equivalence point of the secondary phosphate. The acidvalue indicates the ratio of free acid, calculated as free P₂O₅, to thetotal P₂O₅. The definitions and determination methods for the total P₂O₅and the free P₂O₅ are explained in detail in the publication by W.Rausch “Die Phosphatierung von Metallen”, 1988, pages 299 to 304.

In accordance with the invention it is particularly advantageous whenthe aqueous, phosphate-containing solution contains 0.3 to 3 g Zn²⁺/land 0.1 to 3 g nitroguanidine/l, where the acid value is 0.03 to 0.3 andthe weight ratio of Zn:P₂O₅=1:5 to 1:30. With this inventive solution,which due to its zinc content of 0.3 to 3 g/l is suited for performingthe low-zinc phosphatizing, particularly good results were achieved onthe whole.

In accordance with the invention the aqueous solution should contain 0.5to 20 g NO₃ ⁻/l. The inventive nitrate content favorably influences themaintenance of the optimum coating weight of 1.5 to 4.5 g/m. The nitrateis added to the phosphatizing solution in the form of alkali nitratesand/or by means of the cations present in the system, e.g. as zincnitrate, and/or as HNO₃. Since the nitrate-free aqueous solution alsoprovides good phosphatizing results, the known accelerating effect ofthe nitrate is in all probability of minor importance in the presentcase.

In accordance with the invention it is furthermore provided that thephosphatizing solution contains 0.01 to 3 g Mn²⁺/l and/or 0.01 to 3 gNi²⁺/l and/or 1 to 100 mg Cu²⁺/l and/or 10 to 300 mg Co²⁺/l. These metalions are incorporated in the phosphate coating and improve lacqueradhesion and protection against corrosion.

In accordance with a further aspect of the invention it is provided thatthe aqueous phosphatizing solution contains 0.01 to 3 g F⁻/l and/or 0.05to 3.5 g/l complex fluorides, preferably (SiF₆) or (BF₄). The fluorideis added to the phosphatizing solution when metallic surfaces consistingof aluminum or aluminum alloys should be phosphatized. The complexfluorides are added to the phosphatizing solution in particular forstabilization, so that a longer dwell time of the phosphatizing baths isachieved.

The object underlying the invention is furthermore obtained by employinga process of phosphatizing, where the metallic surfaces are cleaned, aresubsequently treated with the aqueous, phosphate-containingphosphatizing solution for a period of 5 seconds to 10 minutes at atemperature of 15 to 70° C., and are finally rinsed with water. Thisprocess can be performed with simple technical means and is extremelysafe in operation. The phosphate coatings produced by means of thisprocess have a constantly good quality, which does not even decreasewith an extended operating time of the phosphatizing bath. The minimumphosphatizing time is shorter in the process in accordance with theinvention than in known low-zinc processes employing the usualaccelerators. Minimum phosphatizing time is considered to be the time inwhich the surface is covered with a phosphate coating for 100%.

In accordance with the invention it is provided that the treatment ofthe metallic surfaces with the phosphatizing solution is effected byspraying, dipping, spray dipping or roller application. These workingtechniques open a very wide and different range of applications to theprocess in accordance with the invention. In accordance with theinvention it turned out to be particularly advantageous when thephosphatizing solution used for spraying has a weight ratio ofZn:P₂O₅=1:10 to 1:30, and when the phosphatizing solution used fordipping has a weight ratio of Zn:P₂O₅=1:5 to 1:18.

In accordance with the invention it is often advantageous when aftercleaning the metallic surfaces are treated with an activator thatcontains a titanium-containing phosphate. This a supports the formationof a closed, finely crystalline zinc phosphate coating.

Finally, it is provided in accordance with the invention that after therinsing operation following the phosphatizing, the metallic surfaces areaftertreated with a passivating agent. The passivating agents used mayboth contain Cr and be free from Cr.

In the cleaning of the metallic surfaces provided in accordance with theinventive process both mechanical impurities and adhering fats areremoved from the surface to be phosphatized. The cleaning of themetallic surfaces belongs to the known prior art and can advantageouslybe performed with an aqueous-alkaline cleaner. Expediently, the metallicsurfaces are rinsed with water after cleaning. Rinsing the cleaned orphosphatized metallic surfaces is effected either with tap water or withdeionized water.

The phosphatizing solution in accordance with the invention is producedin that about 30 to 90 g of a concentrate containing the inorganiccomponents of the phosphatizing solution as well as water are filled upwith water to 1 l. Subsequently, the provided amount of nitroguanidineis introduced into the phosphatizing solution in the form of asuspension or as powder. The solution is then ready for use, and thesubstances consumed during phosphatizing can continuously be completedby adding the concentrate and the nitroguanidine.

To avoid the difficult dosage of the nitroguanidine as powder, it isprovided in accordance with the invention that the nitroguanidine isintroduced into the aqueous solution in the form of a stabilizedsuspension. In accordance with the invention, the suspension isstabilized with a sheet silicate. This suspension contains 100 to 300 gnitroguanidine/l, 10 to 30 g sheet silicate/l and the rest water. It caneasily be delivered by means of pumps and is stable over 12 months, i.e.the nitroguanidine does not precipitate even after an extended period.The suspension is prepared in that the sheet silicate is suspended in 1l fully deionized water, and then the nitroguanidine is stirred into thesame. At the pH value of 2 to 3 existing in the phosphatizing solution,the suspension is destroyed, and the nitroguanidine is released in afine distribution. In accordance with the invention, the sheet silicates[Mg₆(Si_(7.4)Al_(0.6))O₂₀(OH)₄]Na_(0.6).xH₂O and[(Mg_(5.4)Li_(0.6))Si₈O₂₀(OH₃F)₄]Na_(0.6).xH₂O turned out to beparticularly useful. These are synthesized smectite-type three-layersilicates. The sheet silicates have no disadvantageous effect on theformation of the phosphate coatings. Apart from their actualadvantageous effect they also improve the sedimentation of the phosphatesludge and increase its solids content.

The subject-matter of the invention will subsequently be explained indetail with reference to embodiments.

The embodiments 1 and 2 were performed by means of the following processsteps:

a) The surfaces of metallic articles consisting of steel sheet werecleaned for 5 minutes at 60° C. with a weakly alkaline cleaner (2%aqueous solution) and degreased in particular.

b) Then, rinsing with tap water was effected for 0.5 minutes at roomtemperature.

c) Subsequently, an activation was effected with an activator (3 g/lH₂O) containing a titanium phosphate for a period of 0.5 minutes at roomtemperature.

d) Then, phosphatizing was performed by dipping at about 550° C. for 3minutes.

e) Finally, rinsing was performed with tap water for 0.5 minutes at roomtemperature.

f) The phosphatized surfaces were dried with compressed air.

The composition of the aqueous solutions used for phosphatizing and theproperties of the phosphate coatings are indicated in Table 1.

In accordance with the embodiments 1 and 2, comparative tests were madewith phosphatizing solutions known per se, which contained, however, adifferent accelerator (Comparative Tests A and B). In addition, acomparative test was made with a phosphatizing solution which wasnon-inventive as regards the ratio ZN:P₂O₅, and which containednitroguanidine as accelerator (Comparative Test C). In the ComparativeTests A, B. C the process steps a) to f) were performed. The compositionof the phosphatizing solutions used for the comparative tests and theproperties of the phosphate coatings are indicated in Table 2.

The comparison of the embodiments 1 and 2 with the Comparative Tests A,B and C reveals that with the phosphatizing solution in accordance withthe invention as against the known and well-tried phosphatizingsolutions good results are achieved, but where the nitroguanidine hasmuch better functional properties than the accelerator NO₂ ⁻TheComparative Test C reveals that only by using the inventive parametersgood and practical phosphatizing results are achieved.

The embodiments 3 and 4 were performed by using the following processconditions, where it should in particular be checked whether theinvention was suited for phosphatizing cavities: Steel sheets weretreated in a box simulating a cavity in accordance with process steps a)to e), which were also employed in the embodiments 1 and 2. Drying thephosphatized steel sheets was effected in the cavity (box) at roomtemperature without compressed air. The composition of the aqueoussolutions used for phosphatizing a cavity and the properties of thephosphate coatings are indicated in Table 3.

As regards the coating weight, crystallite edge length and minimumphosphatizing time, the phosphate coatings of the embodiments 3 and 4approximately had the same properties as the phosphate coatings of theembodiments 1 and 2.

In accordance with the embodiments 3 and 4 the Comparative Tests D and Ewere made, where the individual process steps were identical. Thephosphatizing solutions used in the Comparative Tests D and E are knownper se and contain hydroxylamine as accelerator. The composition of thesolutions used for performing the Comparative Tests D and E and theproperties of the phosphate coatings are indicated in Table 4.

A comparison of the embodiments 3 and 4 with the Comparative Tests D andE reveals that with the invention a very good phosphatizing of cavitiescan be achieved, as in accordance with the invention complete, closedphosphate coatings are produced, and there is no formation of flashrust. The term “formation of flash rust” includes that on the metallicsurface which does not have a complete, closed phosphate coating a rustlayer is formed upon drying, which is very disadvantageous. In somecases, there is no formation of flash rust, although there is nocomplete, closed phosphate coating, which should be due to a passivationof the metallic surface by the phosphatizing solution.

For checking the corrosion properties of and the lacquer adhesion onvarious metallic substrates phosphatized in accordance with theinvention lacquer adhesion values were determined.

In Table 5, the lacquer adhesion and corrosion protection values areindicated, which were determined for different sheets (substrates),where the individual substrates in accordance with Examples 5, 6 and 7were phosphatized by dipping with inventive solutions, and thesubstrates in accordance with Comparative Tests F and G werephosphatized by dipping with known solutions. Dipping the individualsubstrates was effected in accordance with the aforementioned processsteps a) to f). The composition of the phosphatizing solutions used forExamples 5, 6 and 7 is indicated in Table 7. There are also indicatedthe compositions of the known phosphatizing solutions used forperforming the Comparative Tests F and G. After phosphatizing thesubstrates by dipping, an electrodeposition paint, a filler and afinishing paint were applied. Subsequently, an outdoor weathering testwas made, evaluated after 6 months, a salt-spray test and a test byflying stones after an alternate climatic test over 12 rounds. In Table5 the subsurface corrosion of the lacquer coating, measured in mm, isindicated, which was determined in the individual tests, where for theflying-stones test the exfoliation of lacquer is indicated in percent.

In Table 6, the lacquer adhesion and corrosion protection values areindicated for various substrates, which were phosphatized by spraying.Spray phosphatizing the substrates was performed in accordance with theinvention by using the following process steps:

g) The surfaces of the substrates were cleaned with a weakly alkalinecleaner (2% aqueous solution) for 5 minutes at 60° C. and degreased inparticular.

h) Subsequently, rinsing with tap water was effected for 0.5 minutes atroom temperature.

i) Then, spray phosphatizing was performed for 2 minutes at 55° C.

k) Subsequently, rinsing was performed with a chromium-free rinsingagent, which contained (ZrF₆)²⁻, at room temperature for 1 minute, so asto passivate the phosphatized substrates.

l) Finally, rinsing was performed with fully deionized water for 1minute at room temperature.

m) The phosphatized substrates were dried in the oven for 10 minutes at80° C.

The compositions of the inventive aqueous phosphatizing solutions, whichwere used for performing Examples 8, 9 and 10, are indicated in Table 8.The composition of the known phosphatizing solution, which was used forcarrying out Comparative Test H, is likewise indicated in Table 8. Onthe substrates phosphatized by spraying, an electrodeposition paint, afiller and a finishing paint were then applied. The phosphatized andpainted substrates were then subjected to an outdoor weathering test for6 months, a salt-spray test, a cross-cut test and an alternate climatictest over 12 rounds and subsequently to flying stones. In Table 6 thevalues determined for the individual substrates are indicated, where forthe cross-cut test a rating is indicated, and for the outdoor weatheringtest, the salt-spray test and the alternate climatic test the subsurfacecorrosion of the lacquer coating is indicated, measured in mm. For theflying stones, the exfoliation of lacquer is indicated in percent.

The protection against corrosion, which is achieved by the inventivephosphatizing process, can be compared with the corrosion protectionachieved by using well-tried, known phosphatizing processes, whichemploy nitrite as accelerator. In the inventive phosphatizing process,however, the use of the accelerator nitrite is avoided, the use of whichincreasingly meets with disapproval, as during phosphatizing reactionproducts are formed from nitrite, which are harmful to the environmentand are in part toxic for man. The lacquer adhesion and anticorrosiveeffect achieved by means of the inventive phosphatizing process must beevaluated as very good to good.

TABLE 1 Example 1 Example 2 Zn²⁺  1.4 g/l  1.4 g/l Mn²⁺  1.0 g/l  1.0g/l Ni²⁺  1.0 g/l — Cu²⁺ —   8 mg/l NO₃ ⁻  3.0 g/l  3.0 g/l PO₄ ³⁻(total) 18.0 g/l 18.0 g/l =P₂O₅ (total) 13.5 g/l 13.5 g/l Nitroguanidine 0.5 g/l  0.5 g/l Na⁺ the amount required for the adjustment oftitration data Acid value 0.09 0.09 Coating weight 2.4 g/m2 2.6 g/m2Crystallite edge length 2-8 μm 2-8 μm Minimum phosphatizing time <60 sec<60 sec

TABLE 1 Example 1 Example 2 Zn²⁺  1.4 g/l  1.4 g/l Mn²⁺  1.0 g/l  1.0g/l Ni²⁺  1.0 g/l — Cu²⁺ —   8 mg/l NO₃ ⁻  3.0 g/l  3.0 g/l PO₄ ³⁻(total) 18.0 g/l 18.0 g/l =P₂O₅ (total) 13.5 g/l 13.5 g/l Nitroguanidine 0.5 g/l  0.5 g/l Na⁺ the amount required for the adjustment oftitration data Acid value 0.09 0.09 Coating weight 2.4 g/m2 2.6 g/m2Crystallite edge length 2-8 μm 2-8 μm Minimum phosphatizing time <60 sec<60 sec

TABLE 3 Example 3 Example 4 Zn²⁺  1.4 g/l  1.9 g/l Ni²⁺  1.0 g/l  1.0g/l Mn²⁺  1.0 g/l  1.0 g/l P₂O₅ (total) 12.0 g/l 12.0 g/l Acid value 0.09  0.09 NO₃ ⁻  3.0 g/l  3.0 g/l Nitroguanidine  0.5 g/l  0.9 g/l Na⁺the amount required for the adjustment of titration data Complete,closed phosphate yes yes coating Formation of flash rust no no

TABLE 3 Example 3 Example 4 Zn²⁺  1.4 g/l  1.9 g/l Ni²⁺  1.0 g/l  1.0g/l Mn²⁺  1.0 g/l  1.0 g/l P₂O₅ (total) 12.0 g/l 12.0 g/l Acid value 0.09  0.09 NO₃ ⁻  3.0 g/l  3.0 g/l Nitroguanidine  0.5 g/l  0.9 g/l Na⁺the amount required for the adjustment of titration data Complete,closed phosphate yes yes coating Formation of flash rust no no

TABLE 5 Lacquer adhesion values, dip application Comparative ExamplesTests Substrate 5 6 7 F G Outdoor weathering for 6 months, mm subsurfacecorrosion, measured unilaterally from the scratch. Steel <1  <1   1.5 <1  2.5 Zinc-plated steel 1  1 1   1.5   2.5 Galvanized steel 0 <1 1  0 <1Steel with Fe-Zn coating <1  <1 <1  <1 <1 AlMgSi, unpolished 3  0 0 <1to 3 — AlMgSi, polished 5 <1 0  4 — Salt-spray test, 1008 h, accordingto DIN 50021 SS, mm subsurface corrosion Steel <1  <1   1.5 <1  1Alternate climatic test over 12 rounds according to VDA 621-415,subsurface corrosion in mm, measured unilaterally from the scratch, andsubsequently flying stones according to specification of VW AG, %exfoliation of lacquer, indicated in () Steel <1(0.5) <1(0.5) 1.5(0.5)<1(1) 2(2) Zinc-plated steel 6.5(1.5) 7(8.5) 7(5) 5.5(2) 8(40)Galvanized steel 1.5(0.5) 2(7) 2(2) 1(0.5) 2.5(15) Steel with Fe-Zncoating 1(0.5) 1(0.5) 1(0.5) 1(0.5) 1(0.5)

TABLE 6 Lacquer adhesion values, spray application Comparative ExamplesTest Substrate 8 9 10 H Outdoor weathering for 6 months, mm subsurfacecorrosion, measured unilaterally from the scratch. Steel <1  1 <1  <1 Zinc-plated steel <1    1.5   1.5   1.5 Galvanized steel 0 0 0 0 Steelwith Fe-Zn coating 0 <1  <1  <1  AlMgSi, unpolished 0 0 0 2 AlMgSi,polished 0 0   2.5 5 Salt-spray test, 1008 h, according to DiN 50021 SS,mm subsurface corrosion Steel <1  <1  <1  <1  Cross-cut test after 240h, according to DIN 50017 KK and DIN/ISO 2409, rating Steel 1 2 1 1Zinc-plated steel 1 1 1-2 1 Galvanized steel 1 1 2 1 Steel with Fe-Znlayer 1 1 1 1 AlMgSi, unpolished 1 0 3 1 AlMgSi, polished 1 0-1 3 1Alternate climatic test over 12 rounds according to VDA 621-415,subsurface corrosion in mm, measured unilaterally from the scratch, andsubsequently flying stones according to the specification of VW AG, %exfoliation of lacquer, indicated in () Steel <1 (2) 1 (5) <1 (2) <1 (2)Zinc-plated steel 5 (5.5) 5.5 (9) 6 (14) 5.5 (4) Galvanized steel 1.5(1) 2.5 (2) 2.5 (1.5) 1.5 (1) Steel with Fe-Zn coating 1 (1) 1 (2) 1 (1)1 (1)

TABLE 7 Substance/ Examples Comparative Tests Value 5 6 7 F G Zn²⁺  1.4g/l  1.4 g/l  1.4 g/l  1.4 g/l 3.5 g/l Mn²⁺  1.0 g/l  1.0 g/l  1.0 g/l 1.0 g/l — Ni²⁺  1.0 g/l — —  1.0 g/l — Cu²⁺ —   8 mg/l — — — NO₃ ⁻  3.0g/l  3.0 g/l  3.0 g/l  3.0 g/l 3.0 g/l P₂O₅ (total) 13.5 g/l 13.5 g/l13.5 g/l 12.0 g/l 5.5 g/l Nitroguanidine  0.5 g/l  0.5 g/l  0.5 g/l —  2 g/l NO₂ ⁻ — — —  170 mg/l — Acid value  0.09  0.09  0.09  0.09 0.35

TABLE 7 Substance/ Examples Comparative Tests Value 5 6 7 F G Zn²⁺  1.4g/l  1.4 g/l  1.4 g/l  1.4 g/l 3.5 g/l Mn²⁺  1.0 g/l  1.0 g/l  1.0 g/l 1.0 g/l — Ni²⁺  1.0 g/l — —  1.0 g/l — Cu²⁺ —   8 mg/l — — — NO₃ ⁻  3.0g/l  3.0 g/l  3.0 g/l  3.0 g/l 3.0 g/l P₂O₅ (total) 13.5 g/l 13.5 g/l13.5 g/l 12.0 g/l 5.5 g/l Nitroguanidine  0.5 g/l  0.5 g/l  0.5 g/l —  2 g/l NO₂ ⁻ — — —  170 mg/l — Acid value  0.09  0.09  0.09  0.09 0.35

What is claimed is:
 1. A process for preparing an aqueous zinc phosphatizing solution for producing phosphate coatings on metallic surfaces of iron, steel, zinc, zinc alloys, aluminum, or aluminum alloys wherein the zinc phosphatizing solution comprises: (1) 0.3 to 5 g Zn²⁺/l; (2) 0.1 to 3 g nitroguanidine/l as an accelerator; (3) stabilizer; (4) phosphate where the acid value is 0.03 to 0.3 indicating the ratio of free acid, calculated as free P₂O₅ to the total P₂O₅, and the weight ratio of Zn to P₂O₅, is 1:5 to 1:30; and (5) balance water, where the solution produces finely crystallite phosphate coatings in which the crystallites have a maximum edge length <15 μm, wherein said process comprises the steps of: (a) preparing a concentrate comprising the Zn²⁺ and the phosphate in water; (b) supplying additional water to the concentrate to form a diluted concentrate so that the Zn²⁺ concentration in the zinc phosphatizing solution will be 0.3 to 5 g/l; (c) preparing a stabilized suspension containing 100 to 300 g of nitroguanidine/l, a stabilizer for the suspension and the balance water by suspending the stabilizer for the suspension in deionized water and stirring the nitroguanidine into the suspension; and (d) introducing the stabilized suspension into the dilute concentrate so that the nitroguanidine concentration in the zinc phosphatizing solution is 0.1 to 3 g nitroguanidine/liter.
 2. The process for preparing an aqueous zinc phosphatizing solution defined in claim 1 wherein according to step (c) the stabilizer is [Mg₆(Si_(7.4)Al_(0.6))O₂₀(OH)₄]Na_(0.6).xH₂O or [(Mg_(5.4)Li_(0.6))Si₈O₂₀(OH₃F)₄]Na_(0.6).xH₂O.
 3. The aqueous zinc phosphatizing solution prepared by the process defined in claim
 1. 4. The aqueous zinc phosphatizing solution defined in claim 3 which further comprises 0.5 to 20 g NO₃ ⁻/l.
 5. The aqueous zinc phosphatizing solution defined in claim 3 which further comprises 0.01 to 3 g Mn²⁺/l and/or 0.01 to 3 g Ni²⁺/l and/or 1 to 100 mg Cu²⁺/l and/or 10 to 300 mg Co²⁺/l.
 6. The aqueous zinc phosphatizing solution defined in claim 3 which further comprises 0.01 to 3 g F⁻/l and/or 0.05 to 3.5 g/l of at least one complex fluoride.
 7. The aqueous zinc phosphatizing solution defined in claim 6 which further comprises (SiF₆)²⁻ or (BF₄)⁻ as complex fluoride.
 8. The aqueous zinc phosphatizing solution defined in claim 3 wherein the stabilizer is [Mg₆(Si_(7.4)Al_(0.6))O₂₀(OH)₄]Na_(0.6).xH₂O or [(Mg_(5.4)Li_(0.6))Si₈O₂₀(OH₃F)₄]Na_(0.6).xH₂O.
 9. A process for phosphatizing a metallic surface of iron, steel, zinc, zinc alloys, aluminum, or aluminum alloys which comprises the steps of: (1) cleaning the metallic surface; (2) following step (1), treating the cleaned metallic surface with an aqueous zinc phosphatizing solution prepared by the process defined in claim 1; and (3) finally rinsing the treated metallic surface with water.
 10. The process for phosphatizing a metallic surface defined in claim 9 wherein according to step (2) the treatment of the metallic surface with the phosphatizing solution is effected by spraying, dipping, spray dipping, or roller application.
 11. The process for phosphatizing a metallic surface defined in claim 10 wherein the phosphatizing solution is sprayed and has a weight ratio of Zn to P₂O₅ of 1:10 to 1:30.
 12. The process for phosphatizing a metallic surface defined in claim 10 wherein the phosphatizing solution is dipped and has a weight ratio of Zn to P₂O₅ of 1:5 to 1:18.
 13. The process for phosphatizing a metallic surface defined in claim 10 wherein following step (1) the cleaned metallic surface is treated with an activator which contains a titanium-containing phosphate.
 14. The,process for phosphatizing a metallic surface defined in claim 10 wherein following step (3) the rinsed metallic surface is treated with a passivating agent.
 15. The process for phosphatizing a metallic surface defined in claim 9 wherein the stabilizer is [Mg₆(Si_(7.4)Al_(0.6))O₂₀(OH)₄]Na_(0.6).xH₂O or [(Mg_(5.4)Li_(0.6))Si₈O₂₀(OH₃F)₄]Na_(0.6).xH₂O.
 16. The process for phosphatizing a metallic surface defined in claim 9 wherein the phosphatizing of the metallic surface takes place prior to painting.
 17. The process for phosphatizing a metallic surface defined in claim 16 wherein the painting is carried out by electro-dipcoating. 